This invention relates to a storing method in a semiconductor memory device comprising an electric charge capture layer provided on a semiconductor substrate through an insulating layer, and in particular to a method of setting in an analog manner a threshold voltage of a semiconductor memory element at a predetermined value.
As this sort of semiconductor memory element, for example, an MNOS (metal-nitride-oxide-semiconductor) element, an MAOS (metal-alumina-oxide semiconductor) element, an MOMOS (metal-oxide-metal oxide-semiconductor) element, etc., are known. These elements all have an electric charge capture layer provided on a semiconductor substrate through an insulating layer. That is to say, this electric charge capture layer, for example, in the MNOS element and the MAOS element, means a layer having an electric charge capture level present in the vicinity of the interface between the nitride and the oxide and between the alumina and the oxide, respectively. In the MOMOS element, the whole metal layer sandwiched by the oxides (insulating materials) forms an electric charge capture layer.
An electric charge is injected into the above said electric charge capture layer through the insulating layer from the semiconductor substrate, and, depending upon the amount of electric charge thus injected, the threshold voltage of the semiconductor memory element is controlled. The electric charge injected cannot escape to the semiconductor substrate due to the existence of said insulating layer and is stored in the electric charge capture layer semipermanently, so that the threshold voltage can be used in a non-volatile memory.
The setting of such a threshold voltage is also called writing, and in the case where this setting value takes only two discrete values, the writing is called digital writing. To the contrary, in the case where the setting value can take any consecutive values, the writing of said threshold voltage is called analog writing.
In the case of digital writing, it suffices to store only two values, for example, "1" and "0", so that, particularly, writing accuracy, etc., is not called in question, but in the case of analog writing, it is necessary to write the setting value with high accuracy.
Now, in the case of storing a threshold voltage Vthl in a semiconductor memory element on the basis of the hysteresis curve, heretofore the writing of the threshold voltage has been carried out by applying a single pulse voltage Vg to the gate of the semiconductor memory element. However, the threshold voltage obtained by this type of writing method, in many cases, deviates from the predetermined value VtHl. The reason for this is not clear, but it appears to be due to the facts that the writing into the semiconductor memory element is not carried out in the stable or equilibrium state of the element, that the hysteresis curve varies completely depending upon the structure of the element in which to store, for example, the thickness of the extremely thin oxide layer, etc., in the case of an MNOS element, and so forth.
Also, even with the general semiconductor technique, it is extremely difficult to manufacture elements of absolutely the same structure, there always existing irregularities between the elements. That is to say, even if, for example, a plurality of semiconductor memory elements were manufactured on the same substrate under the same conditions, irregularities in characteristics between these elements and defectiveness could not be avoided.
As is clear even from the above explanation, in the conventional writing method there is the disadvantage that, for the accurate threshold value writing, a feedback mechanism, a circuit for generating a variable pulse corresponding to a feedback signal, a complicated calculator, etc., are always required.
Further, the inclination of the inclined portion of the hysteresis curve is sharp, so that if the pulse voltage applied changes even a bit, the threshold voltage written comes to change greatly, this being one of the major causes for the writing accuracy to lower.
From these points, the conventional method was disadvantageous in that the writing device becomes large-scaled and also becomes complicated in operation.
Also, the conventional method was limited in use and very inconvenient, since the writing pulse waveform must be changed each time according to the desired threshold voltages.